1. Field of the Invention
This invention relates to switched resistor regulators. More specifically, this invention relates to controlling switched resistor regulators when the transfer function of a component pulse width modulator includes a discontinuity.
2. Description of Related Art
A switched resistor regulator ("SRR") is a type of power supply regulator, characterized by switching a resistor into and out of electrical connection between a power source and a load. SRRs and their operation are fully disclosed in U.S. Pat. No. 4,668,906, issued May 26, 1987 in the name of inventor John P. Ekstrand, and assigned to the same assignee, disclosed in U.S. Pat. No. 4,719,404, issued Jan. 12, 1988 in the name of inventor John P. Ekstrand, and assigned to the same assignee, hereby incorporated by reference as if fully set forth herein, and in co-pending applications titled "SWITCHED RESISTOR REGULATOR WITH DIODE SNUBBER FOR PARASITIC INDUCANCE IN SWITCHED RESISTOR", Ser. No. 102,982, filed Sept. 30, 1988 in the name of inventor John P. Ekstrand, and assigned to the same assignee, and titled "SHUNT SWITCHED RESISTOR REGULATOR WITH DIODE SNUBBER", Ser. No. 103,095, filed Sept. 30, 1988 in the name of inventor John P. Ekstrand, and assigned to the same assignee, all hereby incorporated by reference as if fully set forth herein.
Further information on switched resistor regulators and their use is found in a co-pending application titled "FEEDBACK LOOP GAIN COMPENSATION FOR A SWITCHED RESISTOR REGULATOR", Ser. No. 269,194, filed this same day in the name of inventors John P. Ekstrand and Kevin Holsinger, and assigned to the same assignee, hereby incorporated by reference as if fully set forth herein.
A typical SRR employs a pulse-width modulator ("PWM") to generate a pulse train, which is applied to the switch for switching the resistor into and out of electrical connection. A problem arises in operation of the SRR when it is desired to limit electrical connection of the resistor to a low duty cycle. In such cases, the PWM may generate pulses which are quite narrow in width, but the switch may be unable to switch the resistor into electrical connection for only the short duration of the pulse. This problem may typically arise because the switch being employed is a transistor and the storage charge on the transistor prevents switching the resistor into electrical connection for less than a predetermined minimum period of time, e.g. 8-10 microseconds. This is reflected by an abrupt rise in pulse width between a control voltage which is slightly less than a minimum threshold for the PWM to a control voltage which is slightly greater than the minimum threshold. This abrupt rise is referred to herein as a discontinuity in the transfer function of the PWM.
In such cases, a feedback circuit employed by the SRR for stabilizing the pulse width will be subject to a problem: On the one hand, if the feedback circuit operates with a relatively small time constant, it will shift rapidly back and forth between operating states just below and just above the discontinuity in the transfer function of the PWM; this effect is well known in the art of regulator control. On the other hand, if the feedback circuit operates with a large time constant, it will take substantial time to arrive at a stable state in response to changes in the input control voltage. This effect can degrade the performance of powered systems whose power is being supplied by the SRR.